Twitch responses dependent on external calcium ions in the mouse diaphragm in a potassium-free bathing solution

We examined the effect of external Ca2+ on twitches of mouse diaphragm under the inhibition of Na(+)-K(+)-ATPase in vitro. The muscle was directly stimulated in presence of d-tubocurarine (dTc). K+ removal potentiated the amplitude of twitches with a transient prior reduction. This potentiation depended on external Ca2+. Membrane potentials decreased after removal of both external K+ and Ca2+ and were restored after an addition of Ca2+. The removal of both ions increased the content of Na+ in tissues and decreased K+. These changes were restored to the levels in the K(+)-free bathing solution by the addition of Ca2+. These results imply that Na+/Ca2+ exchange can support twitch contraction under the inhibition of Na(+)-K(+)-ATPase activity.